Fuel tanks, such as those used on commercial vehicles, are subject to a number of safety requirements. One of these safety requirements is the inclusion of a thermal relief system that allows venting of pressure within the fuel tank when the tank reaches a predetermined temperature. One type of thermal relief system is disclosed in U.S. Pat. No. 5,111,837 to Morris, which discloses a thermal relief valve including a ring of fusible material that is cast in situ within a chamber positioned between an abutment member and a plate. At elevated temperatures the fusible material melts to allow the plate and the abutment member to slide relative to each other such that the plate may move outwardly from the vent, thereby releasing pressure within the fuel tank.
There are several disadvantages with such an in situ casting method. In particular, molten fusible material, such as molten metal, is required which requires subjecting workers to the hazards of handling high temperature molten metals. There are costs in generating and maintaining the molten material, as well as the safety equipment that must be purchased to work with such molten material. Additionally, employers generally must monitor the level of toxic metals in employee's bodies to ensure the safety of the process.
The in situ casting method, which involves injecting molten metal into a cavity, could result in air bubbles or an otherwise imperfect fill of the cavity, rendering the cast ineffective. Moreover, an imperfectly filled cavity may not be readily visible or otherwise detectible, such that a defective thermal relief vent may be sold to end consumers and installed on a vehicle. Additionally, the releasable vent plate generally must be held in position before, during and for a short time period after the molten material is poured, so that the molten material will solidify with the plate correctly positioned. The logistics of holding the plate in place before, during and shortly after the molten material is poured adds complexity to the in situ casting process.